Texture mapping is a computer graphics technique which comprises a process of overlaying aerial reconnaissance photographs onto computer generated three dimensional terrain images. It enhances the visual reality of raster scan images substantially while incurring a relatively small increase in computational expense. A frequent criticism of known computer-generated synthesized imagery has been directed to the extreme smoothness of the image. Prior art methods of generating images provide no texture, bumps, outcroppings, or natural abnormalities in the display of digital terrain elevation data (DTED).
In general, texture mapping maps a multidimensional image to a multidimensional space. A texture may be thought of in the usual sense such as sandpaper, a plowed field, a roadbed, a lake, woodgrain and so forth or as the pattern of pixels (picture elements) on a sheet of paper or photographic film. The pixels may be arranged in a regular pattern such as a checkerboard or may exhibit high frequencies as in a detailed photograph of high resolution LandSat imagery. Texture may also be three dimensional in nature as in marble or woodgrain surfaces. For the purposes of the invention, texture mapping is defined to be the mapping of a texture onto a surface in three dimensional object space. As is illustrated schematically in FIG. 1, a texture space object T is mapped to a display screen by means of a perspective transformation.
The implementation of the method of the invention comprises two processes. The first process is geometric warping and the second process is filtering. FIG. 2 illustrates graphically the geometric warping process of the invention for applying texture onto a surface. This process applies the texture onto an object to be mapped analogously to a rubber sheet being stretched over a surface. In a digital map system application, the texture typically comprises an aerial reconnaissance photograph and the object mapped is the surface of the digital terrain data base as shown in FIG. 2. After the geometric warping has been completed, the second process of filtering is performed. In the second process, the image is resampled on the screen grid.
The invention provides a texture mapped perspective view architecture which addresses the need for increased aircraft crew effectiveness, consequently reducing workload, in low altitude flight regimes characterized by the simultaneous requirement to avoid certain terrain and threats. The particular emphasis of the invention is to increase crew situational awareness. Crew situational awareness has been increased to some degree through the addition of a perspective view map display to a plan view capability which already exists in digital map systems. See, for example, assignee's copending application Ser. No. 07/192,798, for a DIGITAL MAP SYSTEM, filed May 11, 1988, issued Oct. 24, 1989 as U.S. Pat. No. 4,876,651 which is incorporated herein by reference in its entirety. The present invention improves the digital map system capability by providing a means for overlaying aerial reconnaissance photographs over the computer generated three dimensional terrain image resulting in a one-to-one correspondence from the digital map image to the real world. In this way the invention provides visually realistic cues which augment the informational display of such a computer generated terrain image. Using these cues an aircraft crew can rapidly make a correlation between the display and the real world.
The architectural challenge presented by texture mapping is that of distributing the processing load to achieve high data throughput using parallel pipelines and then recombining the parallel pixel flow into a single memory module known as a frame buffer. The resulting contention for access to the frame buffer reduces the effective throughput of the pipelines in addition to requiring increased hardware and board space to implement the additional pipelines. The method and apparatus of the invention addresses this challenge by effectively combining the low contention attributes of a single high speed pipeline with the increased processing throughput of parallel pipelines.